Sounds like she has some depression, which often goes hand-in-hand with dementia. Could be a good idea to speak to your physician about some depression medication, as we do want to make sure we treat depression in dementia. The other thing I would recommend is making a daily plan for her: have things to do, places to go, people to see. She probably doesn’t have a reason for getting out of bed, so she feels depressed, and goes back to sleep. Ask her “for her help” to finish household projects and get her engaged in chores and other things to “assist” you.